BAINITE ROD WIRE OR STEEL WIRE FOR WIRE DRAWING AND PROCESS FOR PRODUCING THE SAME

Abstract: A bainite rod wire or steel wire having an excellent workability in wire drawing and a process for producing the same. The wire contains on the weight basis 0.90-1.10 % of carbon, not more than 0.50 % of manganese and not more than 0.40 % of silicon; if necessary 0.10-0.30 % of chromium; not more than 0.003 % of aluminum, not more than 0.02 % of phosphorus and not more than 0.01 % of sulfur; and the balance consisting of iron and inevitable impurities. Further it has a tensile strength and a drawing rate as defined by the following formulae (1) and (2), respectively (1): TS ≦ 85x(C)+60 and (2): RA ≧ -0.875x(TS)+158 ; wherein C represents carbon content (wt%), TS represents tensile strength (kgf/mm²), and RA represents drawing rate (%).

Description: Technical Field This invention relates to bainite wire rod and wire for drawing and methods of producing the same. In this invention, "wire rod," when termed as a product, means wire rod processed for drawing by subjecting it to direct heat treatment immediately after rolling from a steel slab, while, "wire," when termed as a product, means wire subjected to heat treatment in preparation for drawing before drawing or after hot rolling and wire subjected to heat treatment for secondary drawing after being subjected to primary drawing by cold working following hot rolling. Background Art Wire rod and wire are ordinarily drawn into a final products matched to the purpose of use. Before conducting the drawing process, however, it is necessary to put the wire rod or wire in a condition for drawing. In the case of high-carbon steel wire rod or wire, the prior art requires that a mixed texture of uniform, fine pearlite and a small amount of pro-eutectoid ferrite be established before drawing, and, therefore, a special wire rod or wire heat treatment called "patenting" is conducted. This treatment heats the wire rod or wire to the austenite formation temperature and then cools it at an appropriate cooling rate to complete pearlite transformation, thereby establishing a mixed texture of fine pearlite and a small amount of pro-eutectoid ferrite. In the wire rod production method of Japanese Patent Publication No. Sho 60-56215, a heat treatment is conducted for obtaining a mixed texture of fine pearlite and a small amount of pro-eutectoid ferrite by immersing the wire rod heated to the austenite formation temperature in molten salt and then cooling it from 800 - 600°C at a cooling rate of 15 - 100 °C /sec. However, pearlite texture involves the problems of ductility degradation during drawing at a high reduction of area and of cracking in twist test (hereinafter referred to as "delamination"). The object of this invention is to provide bainite wire rod or wire excellent in ductility and not giving rise to the foregoing problems during drawing, and to methods of producing the same. Disclosure of the Invention For achieving this object, the present invention provides bainite-texture wire rod or wire having a chemical composition containing C, Mn, Si, and, if required, further containing Cr in an amount specified by the invention, the upper limit value of P and S content being restricted, and further having prescribed tensile strength and reduction of area. For achieving this object, the present invention also provides bainite wire rod or wire having a diamer of 3.0 - 5.5 mm by increasing the cooling rate up to the nose position in the TTT diagram during cooling of wire rod after hot rolling or during heat treatment of wire after heat treatment at austenite formation temperature, thereby preventing formation of pearlite texture, and then isothermally holding the wire rod or wire at 350 - 500 °C . In other words, following rolling of the wire rod or heating of the steel wire it is cooled from the temperature range of 1100 - 755 °C to the temperature range of 350 - 500 °C at a cooling rate of 60 - 300 °C /sec and maintained at this temperature for at least a specified period to suppress formation of micromartensite texture and thus provide bainite-texture wire rod or wire excellent in drawability, whereby there is obtained wire rod or wire excellent in drawability even at a high reduction of area. The invention is defined in the claims. Brief Description of Drawings Figure 1 is a diagram showing a heat treatment pattern of the present invention. Best Mode for Carrying out the Invention The reasons for the restrictions on the constituent elements of the invention will now be discussed. The reasons for the restrictions on the chemical compositions of the starting steel slab and wire will be described in the following. C is a fundamental element governing strength and ductility, strength increasing with higher carbon content. The lower limit of C content is set at 0.90 wt% for ensuring hardenability and strength and the upper limit is set at 1.10 wt% for preventing formation of pro-eutectoid cementite. Si is added as a deoxidizing agent. Si is also an element which solid-solution hardens the steel and is further capable of reducing wire relaxation. However, increasing the amount of Si reduces the amount of scale formation, degrading mechanical scaling property, and also lowers the lubricity somewhat. The upper limit of Si content is therefore set at 0.40 wt%. Mn is added as a deoxidizing agent. Although Mn is an element which strengthens the steel by its presence in solid solution, increasing the amount added increases the likelihood of segregation at the center portion of the wire rod. Since the hardenability of the segregated portion increases, shifting the finishing time of transformation toward the long period side, the untransformed portion becomes martensite, leading to wire breakage during drawing. The upper limit of Mn content is therefore set at 0.50 wt%. Cr, an element which increases steel strength, is added as occasion demands. While increasing the amount of Cr increases strength, it also increases hardenability and moves the transformation finishing time line toward the long period side. Since this prolongs the time required for heat treatment, the upper limit of Cr content is set at 0.30 wt%, while the lower limit thereof is set at 0.10 wt% for increasing strength. Since P and S precipitate at the grain boundaries and degrade the steel properties, it is necessary to hold their contents as low as possible. The upper limit of P content is set at 0.02 wt% and the upper limit of S content is set at 0.01 wt%. Presence of nonductile inclusions whose main component is Al 2 O 3 , such as Al 2 O 3 , MgO-Al 2 O 3 and the like, is a cause for reduction of ultra-fine wire ductility. In this invention, therefore, Al content is set at not more than 0.003 wt% for avoiding ductility reduction by nonductile inclusions. The rolling conditions and heat treatment conditions for obtaining the bainite wire rod and wire of this invention will now be discussed. The reason for defining the temperature from which cooling is started following wire rod rolling and the wire heating temperature as 755 - 1100 °C is that 755 °C is lower limit temperature of austenitic transformation while abnormal austenite grain growth occurs when the temperature exceeds 1100°C. The reason for defining the cooling rate from the start of wire rod or wire cooling to the isothermal holding temperature range of 350 - 500 °C as 60 - 300 °C /sec is that 60 °C /sec is the lower limit of the critical cooling rate for formation of the upper bainite texture while 300 °C /sec is the upper limit of the industrially feasible cooling rate. The reason for setting the isothermal holding temperature following cooling as 350 - 500 °C is that 350 °C is the lower limit temperature for upper bainite texture formation while 500 °C is the upper limit temperature for upper bainite texture formation. The required isothermal holding time in the temperature range between 350 - 500 °C is calculated from the transformation finishing time line in the TTT diagram. If the immersion time in the cooling tank is insufficient, however, martensite forms and becomes a cause for wire breakage during drawing. Since holding for not less than the finishing time of transformation is therefore required, the holding time in the temperature range of 350 - 500 °C is defined as the time Y sec determined by the following equation (3). Y = exp (19.83 - 0.0329 x T) where T : heat treatment temperature (°C ). The reasons for the limitations on the characteristics of the wire rod and wire having a diameter of 3.0 - 5.5 mm which are products of the invention will now be discussed. Since tensile strength is strongly dependent on C content, it is given in terms of its relationship with C content in the manner of equation (1). In wire rod or wire having bainite texture, the cementite precipitation is coarser than it is in prior art wire rod and wire having pearlite texture and, therefore, the tensile strength is lower for the same composition. In wire-drawing, lowering the initial tensile strength improves the drawability and enables drawing to a high reduction of area. The tensile strength is therefore limited in the manner of equation (1) as the limit up to which the drawability is not degraded. When the upper limit is exceeded, the drawability is degraded, causing the occurrence of breakage or delamination in the course drawing. The reduction of area is an important factor indicative of ease of processing during drawing. Even at the same tensile strength, raising the reduction of area lowers the work hardening rate and enables drawing to a high reduction of area. In wire rod having bainite texture, the cementite precipitation is coarser than it is in prior art wire rod having pearlite texture and, therefore, the reduction of area is higher for the same tensile strength. The reduction of area is therefore limited in the manner of equation (2) as the limit up to which the drawing limit is not degraded. When the lower limit is not reached, the drawability is degraded, causing the occurrence of breakage or delamination in the course drawing. In addition to having the tensile strength and reduction of area prescribed in the foregoing, the invention wire rod or wire having bainite texture further has a microstructure of not less than 80% upper bainite texture in terms of area ratio and an Hv of not more than 450. As a result, its drawability is even further enhanced. EXAMPLES Example 1 Table 1 shows the chemical compositions of tested steel specimens. A - D in Table 1 are invention steels and E and F are comparison steels. Steel E has a C content exceeding the upper limit and steel F has a Mn content exceeding the upper limit. The specimens were produced by casting 300 x 500 mm slabs with a continuous casting machine and then bloom pressing them into 122 - mm square slabs. After these slabs had been rolled into billets, they were rolled into wire rods of the diameters shown in Table 2 and subjected to DLP (Direct Lead Patenting) cooling. The wire rods were drawn to 1.00 mm⊘ at an average reduction of area of 17% and subjected to tensile test and twist test. The tensile test was conducted using the No. 2 test piece of JISZ2201 and the method described in JISZ2241. In the twist test, the specimen was cut to a test piece length of 100d + 100 and rotated at a rotational speed of 10 rpm between chucks spaced at 100d. d represents the wire diameter. The characteristic values obtained in this manner are also shown in Table 2. No. 5 - No. 10 are comparative steels. In No. 5, pearlite which formed because the cooling rate was too slow reduced the drawability, leading to breakage during drawing. In No. 6, pearlite which formed because the isothermal transformation temperature was too high reduced the drawability, leading to breakage during drawing. In No. 7, martensite which formed because the isothermal transformation treatment time was short reduced the drawability, leading to breakage during drawing. In No. 8, bainite texture did not form because the temperature from which cooling was started was too low, reducing the drawability and leading to breakage during drawing. In No. 9, pearlite which formed because the C content was too high reduced the drawability. In No. 10, micromartensite which formed in conjunction with central segregation caused by an excessively high Mn content reduced the drawability. Chemical Compositions of Tested Steel Specimens Symbol Chemical Compositions (wt%) Remark C Si Mn P S Cr Al A 0.95 0.18 0.40 0.006 0.008 - 0.002 Invention B 0.98 0.15 0.30 0.006 0.008 0.19 0.002 Invention C 1.10 0.16 0.39 0.006 0.007 0.21 0.001 Invention D 1.02 0.20 0.35 0.005 0.008 0.21 0.002 Invention E 1.30 0.11 0.40 0.005 0.008 0.11 0.001 Comparison F 0.98 0.30 1.50 0.006 0.007 0.11 0.002 Comparison Example 2 Table 3 shows the chemical compositions of tested steel specimens. A - D in Table 3 are invention steels and E and F are comparison steels. Steel E has a C content exceeding the upper limit and steel F has a Mn content exceeding the upper limit. The wires were transformed to austenitic texture under the conditions shown in Table 4. After heat treatment they were drawn to 1.00 mm⊘ at an average reduction of area of 17% and subjected to tensile test and twist test. The tensile test was conducted using the No. 2 test piece of JISZ2201 and the method described in JISZ2241. In the twist test, the specimen was cut to a test piece length of 100d + 100 and rotated at a rotational speed of 10 rpm between chucks spaced at 100d. d represents the wire diameter. The characteristic values obtained in this manner are also shown in Table 4. No. 1 - No. 4 are invention steels. Since they satisfy all heat treatment conditions of the invention, they can be drawn into wire that does not exhibit delamination even at 1.00 mm ⊘ following drawing. No. 5 - No. 10 are comparative steels. In No. 5, pearlite which formed because the cooling rate was too slow reduced the drawability, leading to breakage during drawing. In No. 6, pearlite which formed because the isothermal transformation temperature was too high reduced the drawability, leading to breakage during drawing. In No. 7, martensite which formed because the isothermal transformation treatment time was short reduced the drawability, leading to breakage during drawing. In No. 8, the bainite texture ratio was zero because the heating temperature was too low, reducing the drawability and leading to breakage during drawing. In No. 9, pearlite which formed because the C content was too high reduced the drawability. In No. 10, pearlite formed and the reduction of area was low because the Mn content was too high, reducing the drawability. Chemical Compositions of Tested Steel Specimens Symbol Chemical Compositions (wt%) Remark C Si Mn P S Cr Al A 0.95 0.18 0.40 0.006 0.008 - 0.002 Invention B 0.98 0.15 0.30 0.006 0.008 0.19 0.002 Invention C 1.10 0.16 0.39 0.006 0.007 0.21 0.001 Invention D 1.02 0.20 0.35 0.005 0.008 0.21 0.002 Invention E 1.30 0.11 0.40 0.005 0.008 0.11 0.001 Comparison F 0.98 0.30 1.50 0.006 0.007 0.11 0.002 Comparison Industrial Applicability As discussed in the foregoing, since the wire rod or wire produced in accordance with this invention can be drawn to an appreciably higher reduction of area than possible by the prior art method, it has improved delamination resistance property. The invention is therefore able to provide bainite wire rod and wire that are excellent in drawability.

Claims: Bainite wire rod or wire having a diameter of 3.0 - 5.5 mm for drawing containing, in weight percent, C : 0.90 - 1.10%, Si : not more than 0.40% and Mn : not more than 0.50%, optionally Cr : 0.10 - 0.30%, is limited to P : not more than 0.02%, S : not more than 0.01% and Al : not more than 0.003%, the remainder being Fe and unavoidable impurities, and having a microstructure of not less than 80% upper bainite texture in terms of area ratio and an Hv of not more than 450 and has tensile strength and reduction of area determined by the following equations (1) and (2), TS ≦ 85 x (C) + 60 RA ≧ - 0.875 x (TS) + 158 where C : carbon content (wt%), TS : tensile strength (kgf/mm 2 ), and RA : reduction of area (%). Bainite wire rod or wire for drawing according to claim 1 characterized in that it contains Cr : 0.10 - 0.30% as an alloying component. A method of producing bainite wire rod for drawing according to cliam 1, with the steps: rolling into wire rod a steel slab of a composition which contains, in weight percent, C : 0.90 - 1.10%, Si : not more than 0.40% and Mn : not more than 0.50%, optionally Cr : 0.10 - 0.30%, is limited to P : not more than 0.02%, S : not more than 0.01% and Al : not more than 0.003%, the remainder being Fe and unavoidable impurities, cooling the rolled wire rod from the temperature range of 1100 - 755 °C to the temperature range of 350 - 500 °C at a cooling rate of 60 - 300 °C /sec, and holding it in this temperature range for not less than a period of Y sec determined by the following equation (3), Y = exp (19.83 - 0.0329 x T) where T : heat treatment temperature (°C ). A method of producing bainite wire rod for drawing according to claim 3 wherein the starting steel slab contains Cr : 0.10 - 0.30% as an alloying component. A method of producing bainite wire having a diameter of 3.0 - 5.5 mm for drawing according to claim 1, with the steps: heating to the temperature range of 1100 - 755 °C wire of a composition which contains, in weight percent, C : 0.90 - 1.10%, Si : not more than 0.40% and Mn : not more than 0.50%, optionally Cr : 0.10 - 0.30%, is limited to P : not more than 0.02%, S : not more than 0.01% and Al : not more than 0.003%, the remainder being Fe and unavoidable impurities, cooling the heated wire to the temperature range of 350 - 500 °C at a cooling rate of 60 - 300 °C /sec, and holding it in this temperature range for not less than a period of Y sec determined by the following equation (3), Y = exp (19.83 - 0.0329 x T) where T : heat treatment temperature (°C ). A method of producing bainite wire for drawing according to claim 5 wherein the starting wire contains Cr : 0.10 - 0.30% as an alloying component.

Inventors: KAWANA AKIFUMI, NISHIDA SEIKI, OBA HIROSHI, OCHIAI IKUO, KAWANA, AKIFUMI, NISHIDA, SEIKI, OBA, HIROSHI, OCHIAI, IKUO
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1994
Main classification: C22C38
IPC codes: C22C38, C21D1, C21D8, C21D9